1. Field of the Invention
The present invention relates generally to an air/fuel ratio control system for an internal combustion engine and more specifically to an air-fuel ratio control system which utilizes the output of a dual oxygen concentration sensor arrangement to achieve feedback control of the fuel supply system.
2. Description of the Prior Art
The use of a so called three-way catalytic converter in an automotive exhaust system is well known. However, in order to achieve the simultaneous reduction of HC, CO and NO.sub.x, it is necessary to maintain the air-fuel mixture supplied to the combustion chamber or chambers of the engine at or very close to the stoichiometric air-fuel ratio (A/F) in order to maximize the conversion efficiency. The use of O.sub.2 sensors for this purpose is also widely known.
However, as the output characteristics of O.sub.2 sensors vary from one sensor to another, a problem is encountered in that the unit to unit deviations in the sensors induce errors in the feedback control of the fuel supply whereby the stoichiometric air-fuel ratio is not maintained in the desired manner and the efficiency of the three-way conversion in the catalytic converter is inhibited.
To overcome this problem is has been proposed in JP-A-58-72674 to use two O.sub.2 sensors which are arranged as schematically illustrated in FIG. 1. As shown in this figure, one sensor 1 is disposed in an exhaust conduit 2 upstream of a 3-way catalytic converter 3 while the other 4 is disposed downstream thereof. The outputs of the two O.sub.2 sensors are fed to a control unit 5 which in turn controls the amount of fuel injected by a fuel injector 6 disposed in the induction system 7 of an engine 8.
Similar arrangements are also disclosed in JP-A-1-113552 and U.S. Pat. No. 3,939,654 issued on Feb. 24, 1976 in the name of Creps.
An example of the control implemented in connection with this type of system is depicted in flow chart form in FIGS. 2 to 4. The routine depicted in FIG. 2 is such as to utilize the output OSR1 of the upstream O.sub.2 sensor to determine a feedback control factor and is run at predetermined intervals (e.g. 4ms) The first step of this routine is such as to determine if conditions (referred to as FRONT O.sub.2 F/B) which permit the use of the upstream side O.sub.2 sensor exist or not.
In the event that such conditions exist, for example: if the temperature of the engine coolant is not below a predetermined level of Tw; the engine is not being cranked/started; the engine has not just been started; the air-fuel mixture is not being deliberately enriched for engine warm-up; the output of the upstream O.sub.2 sensor has not yet switched from one level to another; or the engine is not undergoing a fuel cut, then it is deemed that conditions which enable the use of the sensor exist and the routine should flow to step 1S2. In this step the output OSR1 of the upstream O.sub.2 sensor is subject to A/D conversion, read and the value set in memory. In step 1S3 the instant value of OSR1 is compared with a slice level SL.sub.F (e.g. 0.45 volt) which is selected to represent the stoichiometric air/fuel ratio. In the event that the outcome is such as indicate that OSR1.gtoreq.SL.sub.F (viz., lean) the routine goes to step 1S4 wherein a flag F1 (i.e. F1=0), while in the event that OSR1&gt;SL.sub.F the routine proceeds to step 1S5 wherein flag F1 is set (F1=1).
As will be appreciated flag F1 is such as to indicate if the air-fuel mixture is richer or leaner than stoichiometric value. F1=0=lean, F1=1=rich.
In steps 1S6 to 1S8 the status of F1 for this run is compared with that of the previous one in manner to establish four possible paths for the routine to follow to one of steps 1S9 to 1S12. In these latter mentioned four steps an air/fuel ratio feedback correction factor is subject following methods of derivation:
(i) In the case the routine flows from 1S6.fwdarw. 1S7.fwdarw. 1S9 the air-fuel ratio is indicated as just having undergone a rich.fwdarw. lean change and is derived by incrementing the instant value by a proportional component PL ( = +PL). This tends to incrementally enrich the air/fuel mixture and thus shift the air-fuel ratio stepwisely back toward the stoichiometric value. PA1 (ii) In the case the routine follows a 1S6.fwdarw. 1S7.fwdarw. 1S10 path, the air-fuel mixture is indicated as just having undergone a lean.fwdarw. rich change. Accordingly is derived by decrementing the instant value by a proportional component PR ( = -PR). This tends to stepwisely lean the mixture back from the rich side. PA1 (iii) In the case of a 1S6.fwdarw. 1S8.fwdarw. 1S11 flow, a previously lean condition is again detected and the value of is derived by adding an integrated component IL. This induces the A/F to return gradually toward the rich side. PA1 (iv) In the event of a 1S6.fwdarw. 1S8.fwdarw. 1S11 flow, a previously rich condition is again detected and the value of is derived by subtracting an integrated component IR. This induces the A/F to return gradually toward the lean side.
The flow chart shown in FIG. 3 depicts a routine which utilizes the output of the downstream O.sub.2 sensor for deriving an correction. This routine is run at predetermined intervals of 512 ms (for example). The reason for this relatively long delay between runs is to ensure that the feedback control which is primarily based on the output of the upstream O.sub.2 sensor (which is highly responsive to the changes in A/F) is not dulled by overly frequent application of the output of the downstream O.sub.2 sensor which, due to its position downstream of the catalytic converter, is more remote and much less responsive to changes in the air-fuel mixture being combusted in the combustion chamber(s) of the engine.
At steps 2S21-2S25 the status of the downstream O.sub.2 sensor is checked to determine if the output (REAR O.sub.2 F/B) can be used for feedback control purposes. The output of the downstream O.sub.2 sensor is deemed to be unsuitable for feedback control correction when the conditions which effect the upstream sensor are found to be unsuitable; when the engine coolant temperature is found to be less than Tw (in this case 70.degree. C.)-step 2S22; when the engine throttle opening LL is fully opened (LL=1)-step 2S23; when the engine load/engine speed ratio Qa/Ne&lt;X1-step 2S24; or when in step 2S25 the downstream O.sub.2 sensor is found not to have been activated.
In the event that the appropriate requirements can be met, indicating that conditions wherein the output of the downstream O.sub.2 sensor can relied upon, the routine goes to step 2S26 wherein the output of the same OSR2 is A/D converted, read and set in memory. At step 2S27 the instant value of OSR2 is compared with a slice level SL.sub.R. In this instance the slice level is selected to represent the stoichiometric air-fuel ratio (e.g. 0.55 volt). In the event that it is found that the OSR2.ltoreq.SL.sub.R the air-fuel mixture is deemed to be on the lean side and the routine flows to steps 2S28-2S31. On the other hand, if OSR2&lt;SL.sub.R the mixture is indicated as being on the rich side and the routine is directed to steps 2S32 to 2S35.
It should be noted that as the slice level SL.sub.R is set a little higher than SL.sub.F due to the fact that gases upstream and downstream of the catalytic converter are different and induce the sensors to exhibit slightly different output characteristics and to also allow for the different degradation rates between the two sensors.
At step 2S28 the PL value is incremented by a fixed value .DELTA.PL. At step 2S29 the value of PR is decremented by a fixed value .DELTA.PR. This has the effect of shifting the overall A/F in the rich direction.
At step 2S30 a constant value .DELTA.IL is subtracted from the integrated component IL in order to reduce the amplitude at which increases as a result of the increase of PL in step 2S28. At step 2S31, a constant value .DELTA.IR is added to the integrated component IR in order to reduce the delay with which the output of the upstream O.sub.2 sensor switches from rich to lean, it being noted that this delay is induced by the increase in the PR value in step 2S29.
When the A/F is indicated by the output of the upstream O.sub.2 sensor to be on the lean side, correction control which is implemented in steps 2S28 to 2S31 changes the wave form from that shown in upper half of FIG. 5 to that shown in the lower half of the same figure.
Under the conditions wherein is asymmetrical (e.g. PL=8% and PR=2%) and the intervals between the switches in the sensor output are relatively long, the changes in A/F with respect to the stoichiometric value are or such a large amplitude as to reduce the purifying performance of the catalytic converter.
To overcome this problem the values of IL is modified to reduce the amplitude while the IR value is decreased in order to decrease the delay with which the output of the upstream O.sub.2 sensor switches (viz., reduce the reversing intervals in the feedback control).
The wave form shown in the upper half of FIG. 6 is similarly changed to that shown in the lower half by steps 2S32 to 2S35.
FIG. 4 shows a routine which is run at uniform crankshaft rotation angle intervals (e.g. 30.degree. CA) and which is used to derive the fuel injection pulse width Ti [ms]. The first step 3S31 is such as to derive the basic injection pulse width Tp by table look-up using data which is recorded in terms of engine speed and the engine load. Following this in step 3S32, the sum of a plurality of correction factors (e.g. engine temperature related correction factor KTW) is calculated and at step 3S33 the actual injection pulse width Ti is derived using the equation: EQU Ti=Tp.times.Co.times. +Ts (1)
where Ts denotes the rise time of the fuel injector(s).
In step 3S34 the derived value of Tis is set in memory and used to produce the appropriate injection pulse(s).
However, with this type of arrangement the delay in the response of the downstream O.sub.2 sensor is unchangeably set a relatively large interval with the result that the correction control of the value based on the downstream O.sub.2 sensor cannot take changing conditions into account whereby appropriate correction during acceleration and the like type of transient conditions is impossible.
As a result the above type of control has left a lot to be desired in control accuracy and A/F ratio control.
A second type of previously proposed control is disclosed in flow chart form in FIGS. 7 and 8. The first step of the routine depicted in FIG. 7 is such as to determine if conditions FRONT O2 F/B are such that the output of the front or upstream O.sub.2 sensor can be accepted for control purposes or not. These conditions are for obvious reasons essentially the same as those previously discussed in connection with step 1S1. As in the above case, if the suitable conditions do not prevail then the routine simply goes to across to step 4S10 wherein the value of is arbitrarily set equal to 1.0.
However, in the event that conditions under which the output VFO of the upstream O.sub.2 sensor can be accepted for control purposes exist, the routine goes to step 4S4 wherein a suitable slice level value SL is obtained by look-up. Following this at step 4S3 the instant VFO value is compared with the just obtained SL value in order to determine if the output voltage of the sensor has switched from a maximum level to a minimum one or vice versa. In the event that it is found that VFO.gtoreq.SL, the mixture is deemed to on the rich side. On the other hand, if VFO&lt;SL then the mixture is indicated as being leaner than stoichiometric.
Steps 4S6 to 4S9 the A/F feedback correction factor is derived depending on the outcome of the comparison conducted in step 4S3. As will be apparent, these steps and the manner in which the routine is directed thereto, are the same as disclosed above in connection with steps 1S9-1S12 of the flow chart shown in FIG. 2. Accordingly, redundant disclosure of the same will be omitted for brevity.
FIG. 8 shows a routine in flow chart form which is run at predetermined uniform intervals and which corrects the slice level SL based on the output VRO of the rear or downstream O.sub.2 sensor. The first step (5S21) of this routine is such as to determine if conditions which permit the use of the VRO signal, prevail or not. This determination is carried out in essentially the same manner as disclosed in connection with step 2S21 disclosed above.
In the event suitable conditions are found to be present the routine flows to step 5S22 wherein the value of VRO which has been A/D converted and read into memory, is compared with a slice level SL2 which is selected to correspond to the stoichiometric air-fuel ratio. In the event that is found that VRO&lt;SL2, indicating that the A/F is on the lean side, then the routine goes to step 5S23 wherein the value of SL is decremented by a preset amount. On the other hand, if the VRO.gtoreq.SL2 (indicating a rich mixture) then at step 5S25 the value of SL is incremented by the above mentioned preset amount.
Thus, when the routine flows through step 5S25 the value of the slice level is increased and induces the period for which the A/F stays on the lean side from TL to TL' (see Fig. 9). On the other hand, when the routine flows through step 5S23 the value of SL is decreased and thus induce the tendency for the A/F ratio to remain on the rich side.
The upper half of FIG. 9 depicts the ratio of the time for which the A/F is rich with respect to the time for which it is lean. In order to reduce this ratio the slice level SL is increased in accordance with the output of the downstream O.sub.2 sensor.
However with this type of control, the correction of the slice level based on the output of the downstream O.sub.2 sensor cannot be by performed with sufficiently high efficiency when the front or upstream O.sub.2 sensor exhibits fast response characteristics.
The reason for this is that the wave form of the upstream O.sub.2 sensor output, which is shown in the lower half of FIG. 9, is based on actually measured values (note that the wave form per se is modelled). The response time reduces as the inclination of the leading and trailing edges increases.
When a sensor which exhibits fast response characteristics is used, the ratio H changes at a relatively slow rate when the SL varies at a relatively high rate. Accordingly, the range in which the A/F can shift is narrow and the A/F ratio error absorbing capacity is limited.
Irrespective of the fact that the downstream O.sub.2 sensor exhibits a substantial delay, the correction of the slice level is constant despite changes in the operating conditions. Accordingly, it is difficult to eliminate the A/F errors under all modes of operation. This of course gives rise to an increase in the amount of exhaust emissions.